JP4688652B2 - Color image reproduction method - Google Patents

Description

  According to the present invention, a screen of a plurality of color plates having m (2 ≦ m <n) values is obtained by associating a threshold image matrix for a plurality of color plates with the input continuous tone image data having n-value gradations. The present invention relates to a color image reproduction method in which a color image is reproduced by superimposing a plurality of created color screens, for example, a film setter, a plate setter, a CTP (Computer to Plate) device, a CTC (Computer to Cylinder) device, The present invention relates to a color image reproduction method suitable for application to printing field equipment (output system) such as a DDCP (Direct Digital Color Proof) system, and other ink jet printers or electrophotographic printers.

  Conventionally, in the printing field, when gradation reproduction is realized by area modulation, it is characterized by the number of screen lines, halftone angle, halftone dot shape, and so-called expressing the grayscale by modulating the halftone dot size. In addition to an AM (amplitude modulation) screen, a so-called FM (frequency modulation) screen is employed in which minute dots of a certain size are arranged pseudo-randomly and gradation is expressed by the density of the dots (Patent Document 1).

  In the technique according to Patent Document 1, when a color image is reproduced by superimposing a CMYK four-color screen, a Y-color plate that is a relatively light color uses an FM screen of minute dots, and the Y color The CMK color plate, which is darker than the plate, uses an AM screen with a mesh angle of 30 °, so that the feeling of roughness due to the use of the FM screen is minimized, and the CMK color plate AM screen By optimizing the rosette, it is said that a printed matter with a digital gradation image in which the generation of moire is suppressed can be obtained.

  The meaning of the screen resolution will be described later with reference to Patent Document 2.

  A necessary relationship between the number of screen lines and the screen angle of the AM screen according to the present invention will be described later with reference to Patent Document 3.

JP 11-146189 A (paragraph [0028]) JP 2005-252888 A JP 2002-369017 A

  By the way, recently, there is a market demand for a high-definition color image that has improved detail reproducibility of an image (picture), has a high definition, has a bright color, has a higher saturation, and hardly generates moiré.

  However, in the color image reproduction method using the AM screen for the CMK color plate and the FM screen for the Y color plate according to the above-described prior art, in particular, high-definition and high-saturation of the pattern related to the CMK color plate. Is difficult.

  It is effective to use an FM screen with the CMYK color plate to increase the resolution and saturation of the pattern. However, when the image is reproduced on the FM screen with the usual four-color version of CMYK, it is rough. And unevenness in a uniform density image of a large area may be conspicuous.

  In the four-color version of CMYK, with the AM screen using the normal number of lines, roughness and unevenness are not noticeable, but it is difficult to achieve high-definition and high-saturation of the pattern. Even if the interval is 30 °, the Y color cannot be arranged at an interval of 30 °, so a moire between the Y color and the CMK color remains.

  The present invention has been made in consideration of such problems, and provides a color image reproduction method that can reproduce a color image that can be highly defined and highly saturated, and that can reduce unevenness. The purpose is to do.

  Another object of the present invention is to provide a color image reproduction method capable of reproducing a color image that can achieve high definition and high saturation and can reduce unevenness and roughness.

  Another object of the present invention is to provide a color image reproduction method capable of reducing unevenness in a high-definition AM screen.

  In the present invention, the screen is selected by paying attention to the fact that the visibility, that is, the degree to which the human eye feels bright when viewing the light, differs depending on the color of the light.

  FIG. 1 shows the spectral luminous efficiency normalized by the sensitivity (luminosity) that the human eye feels when shining the light for each wavelength of light by the International Commission on Illumination (CIE). It is also called.) 10 is shown. It can be seen from the visibility characteristic 10 that the visibility has a peak in the vicinity of green with a wavelength of 555 [nm] and is not normally line-symmetric but has a generally distributed characteristic.

  FIG. 2 shows an explanatory diagram in which the visibility characteristic 10 shown in FIG. 1 and the spectral spectrum (reflectance) of each color are superimposed. Although not shown, the reflectance of K color is 0.

  When color reproduction is performed by subtractive color mixture, C, M, and Y color plates (CMY color plate) are generally used in subtractive color mixture. In the printed matter, a K color plate is also used. In the present invention, it is assumed that gradation reproduction is realized by area modulation. In this case, the concept of a color plate (color separation) having high visibility is introduced. A color plate with high visibility (also referred to as a color separation) is a colored portion of a halftone dot (AM screen) or a colored portion of a dot (FM screen) on which a pattern is reproduced for the white color of paper. The one with high contrast.

  Therefore, in the color image reproduction method according to the present invention, m (2 ≦ m <n) values are obtained by associating the input continuous tone image data having n-level gradations with a plurality of color-plate threshold matrixes. The color image reproduction method for producing a color image by creating a plurality of color plate screens and superimposing the produced color plate screens has the following features (1) to (6).

  Feature (1): Among the plurality of color plate screens, a screen having a higher resolution than a screen of another color plate is used for a color screen having high visibility.

  Here, the resolution means the number of lines when the screen is an AM screen, and means the pattern frequency when the screen is an FM screen. The FM screen pattern frequency will be described with reference to JP-A-2005-252888.

  When an FM screen is created using a known algorithm, the number of constituent pixels consists of one pixel or four pixels, such as a 1 (1 × 1) pixel FM screen and a 4 (2 × 2) pixel FM screen. When the dot size is determined like a dot, the array of threshold values constituting the threshold matrix is determined to determine the output quality, and only the dot size is a parameter that determines the quality of the FM screen. For example, if the dot size is determined to be a 3 × 3 pixel FM screen for an output system that cannot stably reproduce the dots of a 2 × 2 pixel FM screen on the highlight side, halftone (10 [ %] To 50 [%]) (referred to as pattern frequency or pattern resolution) and image quality is reduced.

  FIG. 3 shows a dot pattern 1 on the highlight HL side with a dot percentage of 2 × 2 pixels FM screen with a dot percentage of 5 [%] and a dot pattern 2 with a halftone dot with a dot percentage of 50 [%]. In addition, a dot pattern 3 on the highlight HL side with a dot percentage of 3 × 3 pixels FM screen with a dot percentage of 5 [%] and a dot pattern 4 with a halftone dot with a dot percentage of 50 [%] are shown. Yes.

  4 shows a power diagram when FFT (Fast Fourier Transform) is applied to the dot pattern 2 of the 2 × 2 pixel FM screen of FIG. 3, and FIG. 5 shows the power of the 3 × 3 pixel FM screen of FIG. The power figure at the time of applying FFT with respect to the dot pattern 4 is shown.

  In FIG. 3, the dot pattern 2 of the 2 × 2 pixel FM screen has less roughness than the dot pattern 4 of the 3 × 3 pixel FM screen at 50% of the halftone, but the reproducibility to halftone printing. Is bad. On the other hand, at 50% of the halftone, the pattern frequency 6 of the dot pattern 4 of the 3 × 3 pixel FM screen is about 13 [c / mm], and the pattern frequency 5 of the dot pattern 2 of the 2 × 2 pixel FM screen is 5%. It is lower than some 20 [c / mm]. Here, the pattern frequencies 6 and 5 of the peak value are also called peak spatial frequencies fpeak, respectively. It can be said that the lower the pattern frequency, the coarser the resolution of the FM screen.

  That is, the resolution of the FM screen can be expressed by the pattern frequency. Note that the resolution of the AM screen corresponds to the number of screen lines as described above.

  In order to avoid the confusion of the meanings of the terms to be approximated, an output resolution having a concept different from the screen resolution will be described. The output resolution of an output system such as an image setter and a CTP apparatus (hereinafter, the resolution of the output system is referred to as output resolution R) is, for example, 2540 [pixel / inch = DPI (Dots Per Inch)] = 100 [pixel / mm], Alternatively, 2400 [DPI] = 94.488 [pixel / mm]). In this case, when the dot size of the 1 × 1 pixel FM screen is 2540 [DPI], 10 [μm] × 10 [ μm], 2400 [DPI], 10.6 [μm] × 10.6 [μm], and the dot size of the 2 × 2 pixel FM screen is 2540 [DPI], 20 [μm] × 20 [μm], In 2400 [DPI], it is 21.2 [μm] × 21.2 [μm]. That is, the output resolution R and the pattern frequencies 5 and 6 are different.

  The above description is about the pattern frequency representing the resolution of the FM screen, the output resolution, and the like.

  According to the invention having the feature (1), a screen with a coarser resolution than a screen of another color plate is used for a color plate screen having high visibility among a plurality of color plate screens. Since it is used, unevenness is difficult to visually recognize, in other words, a color image with reduced unevenness can be reproduced.

  Feature (2): In the invention having the above feature (1), when the screens of the plurality of color plates are FM screens, unevenness can be reduced and a high-definition and high-saturation color image can be reproduced. .

  Feature (3): In the invention having the above feature (1), even when the screens of the plurality of color plates are AM screens, unevenness is reduced and a high-definition and high-saturation color image can be reproduced. .

  Feature (4): In the invention having the above feature (1), when the color plate screen with high visibility is an AM screen and the other color plates are FM screens, it is difficult to visually recognize roughness and unevenness. By doing so, roughness and unevenness are reduced, and a color image with high definition and high saturation can be reproduced.

  The color plate having high visibility will be described in detail with reference to FIG. 2. A colored portion (drawing portion or blackening portion) of a halftone dot (AM screen) with respect to the white color of paper. It can also be said that a high contrast of colored portions of dots (FM screens) has high visibility.

  In consideration of CMYK colors according to FIG. 2, the K color plate having no reflectance (distribution) at all wavelengths with respect to the white color of paper (reflectance 0 although not shown in FIG. 2) has the highest visibility. The M color plate with a small component of 550 [nm] having high visibility has the second highest visibility, and hereinafter referred to as C color plate and Y color plate, but also when using a plurality of other color plates, The same can be considered.

  For example, when color reproduction is performed including a K color plate such as a three-color plate of CMK or a four-color plate of CMYK, it is preferable to select the K color plate as the color plate having high visibility.

  Feature (5): In any of the above features (1) to (4), a color with high visibility is a K color plate. In this way, unevenness can be reduced by using a K-color screen with high visibility for a screen with a coarse resolution. In the invention having the feature (5) and any one of the features (1) to (3), unevenness can be reduced, and in the invention having the feature (5) and the feature (4), unevenness and roughness can be reduced.

  Further, when a color image is reproduced using a CMY three-color plate that does not include the K-color plate, a screen with a coarse resolution is used as the M-color plate screen having high visibility.

  Furthermore, when a color image is reproduced using the CMYKR (Red) G (Green) B (Blue) color plate, for example, the screens of K color plate, B color plate, and M color plate with high visibility are coarse in resolution. A screen is also included in the present invention.

  Feature (6): In any of the inventions having the above feature (1) or features (3) to (5), when the color plate having high visibility is an AM screen, the screen line number is 175 [LPI (Lines Per Inch)] to 300 [LPI].

  The grounds for numerical limitation of 175 [LPI] to 300 [LPI] will be described.

  In offset printing that is normally used, it is considered that the dot gain amount (half-% on a printing plate and half-% on a printed material) serves as an index representing ease of printing (ease of occurrence of unevenness). That is, when the dot gain amount is large, it is presumed that the area variation amount when a certain variation (change in ink viscosity or the like) occurs is also large.

  This can be explained by considering the perimeter described below.

  The perimeter refers to the sum of the lengths of drawing boundaries (monochrome boundaries) per unit area of a dot pattern (including an AM screen dot pattern and an FM screen dot pattern).

As can be seen from the dot patterns 100 and 104 of the same drawing% (which can be considered as halftone%) in the same area shown in FIGS. 6A and 6B as an example, the length of one side of the dot pattern 100 is L. For example, since eight 1 × 1 pixel dots 102 with a circumference of 4L are included, the circumference is 32L (area is 8L ² ). On the other hand, since the dot pattern 104 includes two 2 × 2 pixel dots 106 having a side length of 2L and a circumference of 8L, the circumference is 16L (area is 8L ² ).

  That is, the total area of the eight dots 102 constituting the dot pattern 100 and the total area of the two dots 106 constituting the dot pattern 104 are the same, but the perimeter is different. In other words, the dot% of the dot pattern 100 and the dot pattern 104 are the same, but the sum of the lengths of the drawing boundaries of the dot pattern per unit area, that is, the perimeter (also referred to as dot perimeter) is the dot pattern. It can be seen that 104 is half the length of the dot pattern 100.

  In this case, for example, when the dot pattern 100 having a peripheral length of 32L is fluctuated by dot gain, for example, the dot% is increased by +10 [%], the dot% of the dot pattern 104 having a peripheral length of 16L is about +5 [%] It is predicted that

  Thus, it is considered that various fluctuation amounts indicating the stability such as the dot gain amount are substantially proportional to the dot pattern perimeter.

  Further, in the halftone dot of the AM screen, various fluctuation amounts indicating the stability such as the dot gain amount are proportional to the screen line number of the halftone dot.

  Generally, when creating a dot pattern with a higher resolution, the number of pixels constituting the dot is reduced, the perimeter is increased, and various stability deteriorates.

  Therefore, if the dot patterns have the same resolution, it can be said that a dot pattern with a short dot perimeter is a high-performance dot pattern.

  In the halftone dots of the AM screen, 175 [LPI] is used as a standard number of lines, but it is said that printing can be performed without any major problem up to about 300 [LPI] if condition management is performed.

  As shown in FIG. 7, the lower the number of lines, the easier it is to print (good printability) and less unevenness. However, when the number of lines is low, the halftone dot structure is visible and the image quality is low. to degrade. Accordingly, it is difficult to produce a high-quality (high definition and high image quality) printed material at 175 [LPI] or less. In general, with a high quality FM screen, the perimeter is equivalent to 400 [LPI] to 600 [LPI] of an AM screen, and printing is difficult (printability is not good).

  Therefore, when the color plate having high visibility is an AM screen, the number of screen lines is preferably about 175 [LPI] to 300 [LPI].

  According to the present invention, it is possible to reproduce a color image that can achieve high definition and high saturation and can reduce unevenness.

  In addition, according to the present invention, it is possible to reproduce a color image that can achieve high definition and high saturation and can reduce unevenness and roughness.

  Furthermore, according to the present invention, it is possible to reproduce a color image by a high-definition AM screen with almost no unevenness.

  Embodiments of the present invention will be described below with reference to the drawings.

  FIG. 8 shows a printing / plate making system 200 to which the color image reproduction method according to the present invention is applied.

  In this printing / plate making system 200, RGB image data captured from a subject (original image) by a digital camera 202 as an imaging device or RGB image captured from an original image by a plate making input device 204 as a scanner (image reading device). Data (or CMYK image data) is supplied to a RIP (raster image processor) 206, and the RGB image data is temporarily converted into CMYK image data.

  In this case, the RIP 206 stores a plurality of color matrix threshold value matrix data for AM screens and FM screens in advance. The RIP 206 includes a personal computer used for setting an output condition for selecting a desired threshold matrix such as screen line number, screen angle, pattern frequency, AM screen or FM screen, and for image confirmation. 205 is connected.

  The RIP 206 compares CMYK image data, which are continuous tone image data each having n values (for example, n = 256), with the corresponding threshold matrix for the CMYK color plate, and compares the values. Conversion into CMYK dot pattern data corresponding to a CMYK color screen having m (2 ≦ m <n) values.

  The CMYK dot pattern data is sent to a so-called DDCP (also referred to as a real net proofer) 210 to create a print proof PRa on paper. This DDCP 210 confirms the presence or absence of noise components and the print quality before applying to the printing press 220. In this case, the printing paper itself may be used as the paper.

  Also, the CMYK dot pattern data is sent from the RIP 206 to the color ink jet printer 230c1 or the color electrophotographic printer 230c2, and the print proofs PRb and PRc can be easily created on the paper.

  Further, the CMYK dot pattern data is sent to an exposure unit 226 which is a film setter or plate setter constituting an output system such as a CTC apparatus. When the exposure unit 226 is a film setter, a film F is created via the automatic processor 228, and the film F is overlapped with a printing plate material for printing plate and exposed by a surface exposure device (not shown). A printing plate PP is created. When the exposure unit 226 is a plate setter, the printing plate PP is directly output through the automatic processor 228. A printing plate material EM or the like is supplied from a magazine 212 of photosensitive material (including a printing plate material) to the exposure unit 226.

  Each plate plate PP of CMYK is mounted on a plate cylinder (not shown) of the K color plate printing unit 214K, the C color plate printing unit 214C, the M color plate printing unit 214M, and the Y color plate printing unit 214Y constituting the printing machine 220. Is done. The printing paper supplied from the printing paper supply unit 216 is overprinted by the K color printing unit 214K, the C color printing unit 214C, the M color printing unit 214M, and the Y color printing unit 214Y. A printed material PM in which a color image is reproduced is obtained. When the printing machine 220 has a CTC device configuration, CMYK dot pattern data is directly supplied from the RIP 206 by communication, and the printing plate material wound around the plate cylinder is exposed, recorded, developed, and directly printed. It is referred to as version PP.

  9 to 11 show output examples of the RIP 206. FIG.

  9 to 11, Black represents a K color screen, Yellow represents a Y color screen, Magenda represents an M color screen, Cyan represents a C color screen, and Gray represents a color image in which CMYK color plates are superimposed. . The halftone% of each CMYK plate is 30 [%], and the gradation n of the input image data is all 256 gradations. Moreover, since all are monochrome displays (the display includes an image on a printed matter or a simulation image on a display), the appearance evaluation (effect) in the monochrome display and the appearance in the color display are different. The evaluation described here is an appearance evaluation in color display.

  In Comparative Example 1 shown in FIG. 9, an FM screen having a high resolution (pattern frequency of about 18 [c / mm]) is used as a CMYK color plate screen. Depending on the output conditions, roughness and unevenness of the K color plate may be noticeable on the color image.

  In Example 1 shown in FIG. 9, in Comparative Example 1, the K color plate having a higher visibility than the CMY color plate having a lower visibility, the line number of 175 [LPI] having a low number of lines (rough resolution), and the halftone angle. A 45 degree AM screen is used. No roughness, unevenness or moire was observed on the color image. The color image has high definition and high saturation.

  FIG. 10 shows the comparative example 1 of FIG. 9 again.

  In Example 2 shown in FIG. 10, in Comparative Example 1, the K color plate having a higher visibility than the CMY color plate having a lower visibility, the lower number of lines (rough resolution) 300 [LPI], and the screen angle. An 82.5 degree AM screen is used. Also in this case, roughness, unevenness, and moire were not recognized on the color image. The color image has high definition and high saturation.

  In Example 3 shown in FIG. 10, in Comparative Example 1, an FM screen having a coarse pattern frequency of about 12 [c / mm] is used for the K color plate having higher visibility than the CMY color plate. . Also in this case, unevenness and moire were not recognized on the color image. The color image has high definition and high saturation.

  In Comparative Example 2 shown in FIG. 11, all 350 [LPI] high-definition AM screens are used as CMYK color plates. Depending on the printing conditions, unevenness may be noticeable in the K color plate. The halftone angles are C color plate = 15 °, M color plate = 45 °, Y color plate = 0 °, and K color plate = 75 °.

In Comparative Example 2, as defined in Japanese Patent Laid-Open No. 2002-369017, at least the mesh angle of the CMK color plate is a non-rational tangent. Then, the screen lines of the CMK color plate so that the period and angle of the moire generated by superimposing the two plates in the CMK color plate substantially match the screen line number and the mesh angle of the remaining one plate of the CMK color plate. Number and mesh angle are set. Further, the screen lines of the three CMK color plates are d1, d2, and d3, and the screen angles are θ1, θ2, and θ3 (θ1 <θ3 <θ2).
d3 · cos θ3 = d1 · cos θ1−d2 · cos θ2
d3 · sin θ3 = d2 · sin θ2−d1 · sin θ1
The relationship is set to be

  The screen line number of the K color plate of Example 4 is coarser to 175 [LPI] than the screen line number 350 [LPI] of the K color plate of Comparative Example 2. At this time, when the line number and the halftone angle of the K color plate of Comparative Example 2 calculated from the above equations are dk and θk, the screen line number and angle of the K color plate of Example 4 are dk / L and θk, respectively. Set (in the fourth embodiment, L = 2).

  Thus, in Example 4 shown in FIG. 11, in Comparative Example 2, the number of lines is as low as 175 [LPI] in the K color plate having higher visibility than the number of lines and resolution of other CMY color plates ( Since an AM screen with a coarse resolution was used, roughness and unevenness were not recognized.

It is explanatory drawing of the spectral luminous efficiency which normalized the sensitivity (visual sensitivity) when a human eye sees light. It is explanatory drawing of the spectral spectrum (reflectance) of each color. It is explanatory drawing which shows the dot pattern of 5% and 50% of the dot percentage of 2 * 2 pixel FM screen, and the dot pattern of 50% of the dot percentage of 3 * 3 pixel FM screen. FIG. 10 is a power diagram when FFT is applied to a dot pattern having a screen percentage of 50% of a 2 × 2 pixel FM screen. FIG. 10 is a power diagram when FFT is applied to a dot pattern in which a halftone dot percentage of a 3 × 3 pixel FM screen is 50%. FIG. 6A is an explanatory diagram for explaining the perimeter of small dots. FIG. 6B is an explanatory diagram for explaining the perimeter of large dots having the same dot percentage as in FIG. 6A. It is explanatory drawing explaining the effect of the increase in the number of lines with respect to a screen line number, and the difficulty of printing qualitatively. 1 is a block diagram showing a printing / plate making system. It is explanatory drawing which shows the screen of the comparative example 1 and Example 1. FIG. It is explanatory drawing which shows the screen of the comparative example 1 and Example 2, 3. FIG. It is explanatory drawing which shows the screen of the comparative example 2 and Example 4. FIG.

Explanation of symbols

10 ... Spectral luminous efficiency (Visibility characteristics)
1, 2, 3, 4, 100, 104 ... dot pattern 5, 6 ... pattern frequency 102 ... 1 x 1 pixel dot 106 ... 2 x 2 pixel dot 200 ... printing / plate making system 204 ... plate making input machine 205 ... personal computer 206 ... RIP (raster image processor)
210 ... DDCP 212 ... Magazine 214K ... K color plate printing unit 214C ... C color plate printing unit 214M ... M color plate printing unit 214Y ... Y color plate printing unit 216 ... Print paper supply unit 220 ... Printer 226 ... Exposure unit 228 ... Automatic developing machine 230c1 ... color ink jet printer 230c2 ... color electrophotographic printer

Claims (5)

Create a screen of multiple color plates with m (2 ≦ m <n) values by associating the threshold data matrix for multiple color plates with the input continuous tone image data having n-value gradations. In a color image reproduction method for reproducing a color image by superimposing a plurality of color plate screens,
Of the plurality of color plates, at least one color plate is an FM screen;
When the resolution of the color plate is defined as the number of screen lines in the AM screen and the pattern frequency in the FM screen, the resolution of the color plate having high visibility among the plurality of color plates is different. A color image reproduction method characterized by using a screen having a coarser resolution than a color plate screen. The color image reproduction method according to claim 1,
The color image reproduction method, wherein the plurality of color screens are FM screens. The color image reproduction method according to claim 1,
The color image reproducing method, wherein the color plate screen having high visibility is an AM screen, and the other color plate screen is an FM screen. In the color image reproduction method according to claim 1 or 3,
When the color plate having high visibility is an AM screen, the number of screen lines of the AM screen is 175 [LPI] to 300 [LPI]. In the color image reproduction method according to any one of claims 1 to 4,
The color image reproducing method, wherein the color plate having high visibility is a K color plate.

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